3.1. Concentration of Toxic Metals and Trace Elements in Liver and Muscle Tissue
Table 3 shows the mean concentrations (mg/kg wet weight) and standard deviations (SD) of the toxic metals and trace elements in the liver and muscle of the analyzed specimens, without geographical distinction.
Liver tissue had the highest concentrations of all the toxic metals analyzed and of almost all the trace elements analyzed, except for Cr, whose highest mean concentrations were recorded in the muscle. The toxic metals and trace elements in the muscle tissue were in the following concentration sequence from highest to lowest: Zn > Fe> Al > Ba > Cu > Sr > Mn > Cr > Li > B > Ni > V > Pb > Co > Cd. In the liver tissue, the sequence of toxic metals and trace elements differs from that of the muscle and was as follows: Fe > Zn > Cu > Al > Sr > B > Li > V > Ba > Cd > Mn > Ni > Pb > Co > Mo > Cr. The Canary Islands are under the phenomena of haze dust due to the fact that this archipelago is close to the Sahara Desert. The sand has high Fe, Cu and Zn concentrations that influence the content of Fe, Cu and Zn of marine organisms like fishes.
The liver had noteworthy concentrations of Fe (221 ± 158 mg/kg wet weight). The liver normally tends to accumulate higher content of Fe. Afonso et al. [
5] reported concentrations of Fe (532.78 ± 295.63 mg/kg ww) in the liver of
Sarpa salpa higher than those found in the present study.
The content of Al (36.2 ± 28.9 mg/kg ww) found in liver tissue is also worth mentioning because it is much higher than that recorded in muscle tissue (1.46 ± 0.78 mg Al/kg ww). Other authors have also found higher Al concentrations in liver than in muscle tissue [
5].
The mean Cd content (1.40 ± 1.93 mg/kg ww) found here in the liver is notable, being much higher than that observed in the muscle tissue (0.007 ± 0.01 mg Cd/kg ww). Other authors have also found higher concentrations of Cd in fish liver of than in fish muscle [
5,
8,
37]. According to the European Commission Regulation (EC) 1881/2006 of 19 December 2006, the maximum permitted content level of Cd in fish intended for human consumption is 0.05 mg/kg of fish. Therefore, the mean average content level of Cd found in the present study in muscle tissue is below this limit, meaning it is suitable for human consumption [
38]. However, considering the above guideline, the liver would not be suitable for consumption.
The concentration of Pb recorded in liver tissue (0.43 ± 0.59 mg Pb/kg ww) is much higher than that found in muscle tissue (0.03 ± 0.01 mg Pb/kg ww). The European Commission regulation (EC) 1881/2006 of 10 December 2006 sets a legal maximum limit of Pb at 0.3 mg/kg of fish [
38]. The concentration levels determined here in the muscle are below the maximum permitted limit, while the mean Pb concentration found in the liver tissue considerably exceeds this limit, and, therefore, its consumption is not recommended. These data agree with those obtained by other authors who have reported higher concentrations of Pb in the liver than in muscle tissue [
5,
39].
3.2. Concentration of Toxic Metal and Trace Elements in Each Sampling Zone
Table 4 shows the mean concentrations (mg/kg ww) and standard deviations (SD) of the toxic metals and trace elements in each of the sampling areas.
In muscle tissue, the Zn content (17.4 ± 1.18 mg/kg ww) found in the specimens from the south zone is noteworthy, while in the specimens from the north zone, the mean average level of Fe (12.5 ± 11.1 mg/ g ww) is higher than the Fe level of the south and metropolitan areas.
The mean level of Al (1.69 ± 1.18 g/kg ww) in the muscle tissue of the specimens from the metropolitan zone is noteworthy. In specimens such as brook trout, muscle tissue is reported to accumulate less Al than other tissues [
40], as has been verified in the results obtained in the present study in
Sarpa salpa.
The statistical study by zones shows the existence of significant differences in muscle tissue (
p < 0.05) in the Cd content between specimens from the north zone and south zone with specimens from the metropolitan zone; this is likely due to the fact that the specimens from south zone were the biggest (24.8 ± 1.87 cm), indicating that they accumulated high Cd concentrations [
29].
The B content was statistically different among the three areas, with the B content being high in specimens from the south zone. Regarding the Co content, there are significant differences (p < 0.05) between the north and metropolitan zone with the south zone, while the mean Cu level differs statistically in the north and south zone compared to the metropolitan zone. The V content differs statistically between the metropolitan and south zone with the north zone. These differences are likely due to several factors like different ocean currents, the pollution of each zone, temperature, etc. In liver tissue, the content of Fe (301 ± 181 mg/kg ww), Cu (87.0 ± 112 mg/kg ww) and Al (57.7 ± 30.6 mg/kg ww) in specimens from the metropolitan zone stand out. It should be noted that, in fish, the tissue concentration of Al is related to the quality of the water. Higher concentrations were recorded in the liver in the metropolitan zone, as this is the most industrialized zone, while the mean Zn level (113 ± 105 mg/kg ww) in the specimens from the north zone is notable.
In the case of the Al concentration in the liver, there are significant differences (p < 0.05) between the metropolitan zone, where the highest Al (57.7±30.7 mg/kg) level was recorded, and the south and north zones. The metropolitan zone is under anthropic activities because it is the area with the highest population, and it industrialized and a zone with an important transit for large cruise ships, oil and cargo ships. The content of B, Ba and Pb in the south and north zones is statistically different from the content of these trace elements found in the metropolitan zone. The content of Fe and Cr in specimens from the metropolitan zone differ statistically from the content of Fe and Cr found in specimens from the north zone. The highest Fe and Cr concentrations were found in the metropolitan and south zones, respectively, while the Sr and V contents of the specimens caught in the metropolitan zone are statistically different from the content observed in those from the south zone. The highest Sr level was recorded in the metropolitan zone, which is the most polluted zone because of its industrial activity and high-density population.
It should be noted that the lowest mean concentrations of the analyzed elements in liver tissue were recorded in the north zone specimens. The north zone is under a lower anthropic activity, and the pollution is this area is low as well as the population density and the industrial activities. However, in muscle tissue, the specimens from the north zone had the highest concentrations of elements like Fe, Cr, Cu and B.
The correlation study (
Table 5) based on the Spearman correlation coefficient showed a negative correlation between the length and weight of the captured specimens for Pb content. In other words, the greater the size and weight of the specimen, the lower the Pb content in both muscle and liver. It should be noted that Pb can compete with Ca at the binding sites in hydroxyapatite crystals, which are fixed to the bone [
39,
41]. It was found that, in the largest specimens from the north and south zones, Pb probably is fixed in the bones, while in the metropolitan zone, young specimens had higher concentrations of Pb accumulated in soft tissues (liver and muscle) [
39]. The metropolitan zone, located between Anaga and Candelaria, is the most industrialized area, as it includes the Caletillas thermal power station and the port of Santa Cruz de Tenerife, which has a large influx of ships and large vessels. Therefore, the highest concentration of Pb is due to both physiological factors and increased environmental contamination.
On the other hand, the Spearman correlation coefficient showed a positive correlation between length and weight with respect to the concentration of Cd. That is, the greater the size and weight of the specimens, the higher the Cd content. The specimens from the metropolitan zone had lower levels of Cd since they are young specimens of smaller size and weight, while the specimens from the north and south zones, being larger and heavier, had higher Cd concentrations.
3.3. Evaluation of Dietary Intake of Toxic Metals and Trace Elements
Table 6 shows the estimated daily intake (EDI) values (mg/day) of the toxic metals and trace elements analyzed and the percentages of contribution to the respective maximum intakes set by various institutions.
In the case of Cd, considering a consumption of 100 g of salema liver per day, the percentage of contribution to the tolerable weekly intake (TWI), established at 2.5 µg/kg body weight/week [
42], is much higher than the fixed value, which is 572% for adults with an average weight of 68.48 kg. Likewise, the consumption of 100 g of liver per day represents a percentage of contribution to the tolerable daily intake (TDI) of Pb, set at 0.5 µg/kg body weight/day [
41], of 117% for adults weighing 68.48 kg on average. On the other hand, although they are not exceeded, the percentages of contribution to the maximum value (UL) set by the IOM (Institute of Medicine) [
21] for Fe (49.1%, adults), Cu (47%, adults) and Mo (40.0%, adults) from the consumption of 100 g/day of liver are significant. Fish liver consumption has health benefits because it is rich in polyunsaturated fatty acids [
47]. Brustad et al. [
48] studied the vitamin D status in a rural population of northern Norway with high fish liver consumption and concluded that the consumption of fish liver offers a high vitamin D intake. The Chalmers University of Technology research concluded that fatty fish without environmental pollutants protect against type 2 diabetes [
49]. However, in the case of our study, salema liver consumption is not recommended as it represents a significant health risk due to its possible toxic effects because of its toxic metal levels.
On the other hand, the consumption of 100 g of salema muscle per day does not entail any health risk, since the contribution percentages obtained are well below 100% of the maximum value established for both toxic metals and trace elements.